Microelectronic devices can be classified to volatile and nonvolatile devices based on their power characteristics. In volatile devices, the device's states are supported by the electrical power, and the device behaves as expected as long as the circuit receives power. In contrast, in nonvolatile devices, the device's states are stable with or without the applied power, and therefore when the power is off, the device stays in their states without any changes.
The major difference between a volatile and a nonvolatile device is the fundamental designed states of the device. If the device states are stable without any power source, the device is nonvolatile. If the device states require power to maintain, the device is volatile. Nonvolatility is much more desirable than volatility due to the lower power consumption, and the ability to remember and retain information without external power sources.
An example of volatility and nonvolatility is memory devices. A DRAM (dynamic random access memory) is a volatile memory device because the DRAM states are represented by a collection of charges, stored in a capacitor. Because of the inherent leakage of the capacitor charge, the DRAM state where the capacitor is charged is not stable without power. Thus by designing the electron charges as the memory state, the DRAM memory cell is inherently a volatile device. A RRAM (resistive random access memory) is a nonvolatile memory, employing a class of memory materials that have electrical resistance characteristics changeable by external influences. The RRAM memory is represented by the multistable states of high resistance and low resistance, where the applied power is only needed to switch the states and not to maintain them. The examples of such memory materials are perovskite materials exhibiting magnetoresistive effect or high temperature superconducting effect, disclosed in U.S. Pat. No. 6,204,139 of Liu et al., and U.S. Pat. No. 6,473,332 of Ignatiev et al., hereby incorporated by reference.
Another example of volatile device is electro-optic systems for high speed optical data transfer and processing, using electric fields to control the propagation of light through their optical materials. Common electro-optic systems are currently based on devices fabricated in bulk LiNbO3 crystals which have proven maturity and long term stability. The design and selection of current electro-optic media such as LiNbO3 lead to the inevitable feature of volatility, since the current electro-optic media require the presence of electric field to maintain their optical states.
The present invention addresses the nonvolatility of the electro-optic properties in the field of light transmission. The first step in designing nonvolatile electro-optic device is to identify multistable states and multistable medium for optical applications.